fluent/fluentd

Buffer size metrics showing negative values

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#5,303 opened on Sep 24, 2020

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bughelp wanted

Description

Details

I have Fluentd set up to forward logs to Kafka. In some of the Prometheus metrics for the buffers of those Kafka output plugins, I'm seeing negative values. Specifically, I'm seeing negative values for the following metrics:

  • fluentd_output_status_buffer_total_bytes
  • fluentd_output_status_buffer_stage_byte_size

Eg.

fluentd_output_status_buffer_total_bytes{worker_id="3",plugin_id="kafka_output",type="kafka2"} -7897834.0
fluentd_output_status_buffer_stage_byte_size{worker_id="3",plugin_id="kafka_output",type="kafka2"} -7897834.0

This appears to be the same issue as fluent/fluent-plugin-prometheus#55, open since 2018.

Prometheus Config

<source>
  @type prometheus
  port 24240
  metrics_path /metrics
  aggregated_metrics_path /aggregated_metrics
</source>

<source>
  @type prometheus_output_monitor
</source>

Env Info

  • Fluentd: 1.11.2
  • fluent-plugin-prometheus: 1.8.2
  • fluent-plugin-prometheus_pushgateway: 0.0.2
  • fluent-plugin-kafka: 0.14.1
  • Note that Fluentd has multi-process workers enabled

Contributor guide

Tech stack
rubyprometheuskafka
Domain
backendobservability
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
half day
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Fluentd buffer internalsPrometheus metrics understanding
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
40
Research direction
Investigate the computation of buffer size metrics in Fluentd's output plugin, specifically in fluent plugin prometheus. The bug may be due to integer overflow or incorrect subtraction in buffer statistics. Check the referenced issue #55 and examine the code in lib/fluent/plugin/out buf perf.rb or similar files where metrics are emitted. Look for how buffer total bytes and buffer stage byte size are calculated and ensure they are unsigned or properly handled.